This invention relates to frequency modulated phase-locked loops, and more particularly, to audio compensation circuits utilized in frequency modulated phase-locked loops.
A convenient and common method used in the past to provide angular, i.e., frequency or phase, modulation of a carrier has been to modulate the voltage controlled oscillator (VCO) input of a phase-locked loop (PLL) with a modulating signal. The frequency of the modulating signal is greater than the upper cutoff frequency of the phase-locked loop which contains the VCO, and therefore does not affect the stability of the output frequency of the PLL which is locked to the reference oscillator. An AC coupled modulating voltage applied to the VCO provides a symmetrical deviation about this carrier in most cases.
While the output frequency of the PLL is held within very tight limits by the reference frequency, the deviation of the modulating signal about the carrier is not part of the closed loop system and unfortunately varies directly with the nonlinearities of the VCO. Several methods have been tried in the past to compensate for the nonlinearities in the VCO. One method involves the use of an operational amplifier with a multiplicity of feedback resistors each selectively switched into the circuit for a given frequency band to provide discrete amplification factors for each of a plurality of frequency bands of the PLL. However this and other methods have suffered from the drawbacks of requiring an appreciable number of parts and a comparatively large amount of power.
Therefore it can be appreciated that an audio compensation circuit for use in a frequency modulated PLL which involves only a few parts and the minimum of operating current is highly desirable.